1. Field of the Invention
The present invention relates to an alumina insulating substrate, to which copper foil is applied, for mounting semiconductor circuit elements such as an inverter of a motor control or air conditioner, or a power module used in NC control, or example.
2. Discussion of the Related Art
Electronic technology has advanced remarkably during the past decade, with rapidly increasing levels of integration and decreasing cost. The primary reason for this trend is that semiconductors have become the primary electronic device. The trend in the area of power modules is to decrease electronic component size and to include more circuitry in a single package. To achieve this goal, circuit elements are mounted on an insulating substrate such that individual circuit elements are electronically isolated from one another. Metal foil is applied to the flat surfaces of the insulating substrate to which electronic components may be soldered. The substrate is usually made of a ceramic material, such as alumina [Al.sub.2 O.sub.3 ]. However, as electronic devices have become smaller and more powerful, substrate material having higher thermal conductivity for improved heat dissipation is required.
The thermal conductivity of Al.sub.2 O.sub.3 is 17 W /[m.k]. The thermal conductivity of aluminum nitride [AlN] is about 5 times greater, 80-140 W/[m.k ]. As a result, an AlN substrate offers higher heat dissipation than an Al.sub.2 O.sub.3 substrate. However, due to the high cost of AlN, its use has been limited to special applications.
One method for increasing the heat dissipation of an Al.sub.2 O.sub.3 substrate is to decrease the substrate's thickness. However, as the material's thickness is decreased, its stress resistance also decreases. For example, a conventional Al.sub.2 O.sub.3 substrate 0.635 mm thick has a bending strength of 8 kg. If the substrate's thickness is reduced to 0.275 mm, the bending strength is only 2 kg, a reduction of 75%. In such a substrate, there is a high probability of cracks developing over time as a result of thermal stress in the assembly.